For industrial radiography a normal processing cycle is characterised by the following steps: transport of the film through the developer at 28.degree. C., transport through the fixer at 26.degree. C., transport through a rinsing bath and transport through the drying station. If an automatic processing machine is used film transport is made possible by the racks each of which is provided with a lot of rollers immersed in the different processing baths. Due to the normal use of this automatic processing machine the said different processing baths become inevitably polluted by e.g. dust being carried into the processor by the film to be processed itself. Another possibility consists in the generation of very small metallic silver particles in the developer, due to the development process. Inevitably quite a lot of manipulations like an arrest in development, the start of the circulation of processing and regeneration liquids make the generated solid particles become deposited onto the rollers of the racks.
When a film is introduced into the processor as first film of a whole series of films, its first contact with the first stained roller releases the deposit from the said roller or disturbs the deposited layer. As a consequence after one rotation of the said roller the unevenly distributed dirt or stain comes into contact again with the transported film surface so that it may be deposited onto said surface. This phenomenon is recurrently repeated, not only at the first roller, but also at the further rollers that are mounted onto the racks.
The result of this process is that one or more visibly appearing disturbing lines may be observed at the surface of the processed film. This artefact is called "pi-line" as it is recurrently depicted at a distance corresponding to the circumference of the rollers. As the processing is further continuing the artefact may disappear.
The gravity of the said artefact is strongly dependant on the maintenance of the processor and on the silver content of the film. Strongly polluted processors may show many "pi-lines" at one or more successive film sheets when the said processors are started up.
The "pi-line" phenomenon, in some references also called "pie line", has already been described in "Radiographic Artefacts" by Richard J. Sweeney, Ed. J. B. Lippincott Co., Philadelphia, 1983, ISBN 0-397-50554, p. 288 and in "Radiographic Imaging" by Derrick P. Roberts & Nigel L. Smith, Ed. Churchill Livingstone, Edinburgh, 1988, ISBN 0-443-03061-8, p. 162.
From the patent literature it is well-known that the addition of several different additives to the developer as well as to the fixer have been proposed. U.S. Pat. No. 3,515,556 suggests the addition of a mixture of copolymers of catechine and leucocyanidine to the developer or to the fixer. U.S. Pat. No. 4,310,622 looks for a solution of "stripe silver contamination", to be interpretable as "pi-line", by means of the addition of a sulphonated benzimidazole compound.
TETENAL is offering since quite a lot of years an additive comprising a disulphide containing compound as an active substance therein.
The reduction of silver sludge in processing baths has futher been described in U.S. Pat. No. 4,141,734 by Ciba-Geigy, which firm has published the use of these products under the trade name IRGAFORM 1007.
The addition to the developer of a lot of different heterocyclic mercapto compounds and a phosphate ester surfactant has been proposed in EP 223 883.
A more mechanical solution for this problem has been proposed in U.S. Pat. No. 4,853,728 wherein an optimization of the roller configuration of the racks prevents the occurrence of "pi-lines". Otherwise in the processors KODAK MODEL B, trade name product from Eastman Kodak and STRUCTURIX NDT-3, trade name product from Agfa-Gevaert, brush rollers installed at the end of the water rinsing station make the artefact to be removed mechanically before introduction into the drying station.
Another attempt has been made in EP-A 518 627 by coating the silver halide photographic material to be processed with a particulate wax dispersed in a hydrophilic colloid and wherein development proceeds about as described in EP 223 883, mentioned hereinbefore.
Nevertheless the proposed solutions are insufficient, especially for films with a high coating amount of silver, like industrial X-ray films where the appearance of one or more "pi-lines" may be intolerable as the detection of defects in e.g. weldings from pipelines or nuclear application may be covered and thus hidden by the said "pi-lines". Even mechanical means as brush rollers are not able to remove all of the deposit and have frequently to be replaced by new ones. Also from an economical point of view the additional costs resulting therefrom are intolerable. Otherwise additives to the developer not only show an insufficient removal of the artefact, but some of them cause an unpleasant smell and the formation of scum.